As important factors governing properties of magnetic steel strips, there are amounts, sizes, morphology and distribution of AlN and MnS which precipitate in steel. They do not only influence the magnetic properties of final products but also play important roles for the formation of the microstructure of the steel strips during a series of processing.
In the case of grain oriented silicon steel strips, the precipitates such as AlN and MnS are effectively utilized as inhibitors which control a secondary recrystallization. However, with respect to the non-oriented silicon steel strips, there have been disclosed technologies to make the precipitates harmless, as follows:
1. The slab is heated at low temperature so as to check resolution of AlN or MnS (e.g. Patent Publication No. 50-35885).
2. The amounts of S and O are decreased which produce fine precipitates of non-metallic inclusions (e.g. Patent Publication No. 56-22931).
3. Ca and REM are added to control morphology of sulfide inclusions (e.g. Patent Publications No. 58-17248 and No. 58-17249).
4. The steel strip is coiled at ultra high temperature after hot rolling so as to cause a self-annealing thereof, so that AlN is coarsened by self-annealing effect (Patent Publication No. 57-43132).
Most of these technologies are based on the premise of the conventional processes which consist of slab reheating and hot rolling. However, taking into consideration the employment of direct rolling, which is regarded as promising in terms of energy- and process-savings, the above technologies alone are insufficient to obtain the excellent magnetic properties, because in the direct rolling, AlN or MnS finely precipitate in steel during the hot rolling process.
Therefore from the viewpoint of solving the above problems, as a method of coarsening AlN in HDR, technologies have been proposed to coarsen AlN by briefly heating the slab on the way of HDR as taught in Patent Publications No. 56-18045, No. 56-33451 and Laid-Open No. 58-123825. However, these techniques cause non-uniform precipitation of AlN in the thickness direction of the slab. Therefore those methods are not always sufficient for manufacturing the magnetic steel strip of which uniformity of the property is important.